Soil Taxonomy 2nd Ed.1999

Identification of the Taxonomic Class of a Soil 

The taxonomic class of a specific soil can be determined by using the keys that follow in this and other chapters. It is assumed that the reader is familiar with the definitions of diagnostic horizons and properties that are given in chapters 3 and 4 of this publication and with the meanings of the terms used for describing soils given in the Soil Survey Manual. The Index at the back of this publication indicates the pages on which definitions of terms are given. 

Standard rounding conventions should be used to determine numerical values. 

Soil colors (hue, value, and chroma) are used in many of the criteria that follow. Soil colors typically change value and some change hue and chroma, depending on the water state. In many of the criteria of the keys, the water state is specified. If no water state is specified, the soil is considered to meet the criterion if it does so when moist or dry or both moist and dry. 

All of the keys in this taxonomy are designed in such a way that the user can determine the correct classification of a soil by going through the keys systematically. The user must start at the beginning of the “Key to Soil Orders” and eliminate, one by one, all classes that include criteria that do not fit the soil in question. The soil belongs to the first class listed for which it meets all the required criteria. 

In classifying a specific soil, the user of soil taxonomy begins by checking through the “Key to Soil Orders” to determine the name of the first order that, according to the criteria listed, includes the soil in question. The next step is to go to the page indicated to find the “Key to Suborders” of that particular order. Then the user systematically goes through the key to identify the suborder that includes the soil, i.e., the first in the list for which it meets all the required criteria. The same procedure is used to find the great group class of the soil in the “Key to Great Groups” of the identified suborder. Likewise, going through the “Key to Subgroups” of that great group, the user selects as the correct subgroup name the name of the first taxon for which the soil meets all of the required criteria. 

The family level is determined, in a similar manner, after the subgroup has been determined. Chapter 21 can be used, as one would use other keys in this taxonomy, to determine which components are part of the family. The family, however, typically has more than one component, and therefore the entire chapter must be used. The keys to control sections for classes used as components of a family must be used to determine the control section before use of the keys to classes. The descriptions and definitions of individual soil series are not included in this text. Definitions of the series and of the control section and examples of the application are given in chapter 21. The classification of the series and the list of families and their included series for the soils of the 50 States, Puerto Rico, and the Virgin Islands are given in another publication (Soil Series of the United States, Including Puerto Rico and the Virgin Islands: Their Taxonomic Classification). That publication does not include the descriptions or definitions of the series, but descriptions of specific series are available on request from the Natural Resources Conservation Service. No one publication includes descriptions of all the series. 

In the “Key to Soil Orders” and the other keys that follow, the diagnostic horizons and the properties mentioned do not include those below any densic, lithic, paralithic, or petroferric contact. The properties of buried soils and the properties of a surface mantle are considered on the basis of whether or not the soil meets the meaning of the term “buried soil” given in chapter 1. 

If a soil has a surface mantle and is not a buried soil, the top of the original surface layer is considered the “soil surface” for determining depth to and thickness of diagnostic horizons and most other diagnostic soil characteristics. The only properties of the surface mantle that are considered are soil temperature, soil moisture (including aquic conditions), any andic or vitrandic properties, and family criteria. 

If a soil profile includes a buried soil, the present soil surface is used to determine soil moisture and temperature as well as depth to and thickness of diagnostic horizons and other diagnostic soil characteristics. Diagnostic horizons of the buried soil are not considered in selecting taxa unless the criteria in the keys specifically indicate buried horizons, such as in Thapto-Histic subgroups. Most other diagnostic soil characteristics of the buried soil are not considered, but organic carbon if of Holocene age, andic soil properties, base saturation, and all properties used to determine family and series placement are considered. 

Key to Soil Orders 

A. Soils that have:

1. Permafrost within 100 cm of the soil surface; or

2. Gelic materials within 100 cm of the soil surface and permafrost within 200 cm of the soil surface. Gelisols, p. 445 

B. Other soils that:

1. Do not have andic soil properties in 60 percent or more of the thickness between the soil surface and either a depth of 60 cm or a densic, lithic, or paralithic contact or duripan if shallower; and

2. Have organic soil materials that meet one or more of the following:

a. Overlie cindery, fragmental, or pumiceous materials and/or fill their interstices1 and directly below these materials, have a densic, lithic, or paralithic contact; or

b. When added with the underlying cindery, fragmental, or pumiceous materials, total 40 cm or more between the soil surface and a depth of 50 cm; or

c. Constitute two-thirds or more of the total thickness of the soil to a densic, lithic, or paralithic contact and have no mineral horizons or have mineral horizons with a total thickness of 10 cm or less; or

d. Are saturated with water for 30 days or more per year in normal years (or are artificially drained), have an upper boundary within 40 cm of the soil surface, and have a total thickness of either:

(1) 60 cm or more if three-fourths or more of their volume consists of moss fibers or if their bulk density, moist, is less than 0.1 g/cm3; or

(2) 40 cm or more if they consist either of sapric or hemic materials, or of fibric materials with less than three-fourths (by volume) moss fibers and a bulk density, moist, of 0.1 g/cm3 or more. Histosols, p. 473 

C. Other soils that do not have a plaggen epipedon or an argillic or kandic horizon above a spodic horizon, and have one or more of the following:

1. A spodic horizon, an albic horizon in 50 percent or more of each pedon, and a cryic soil temperature regime; or

2. An Ap horizon containing 85 percent or more spodic materials; or

3. A spodic horizon with all of the following characteristics:

a. One or more of the following:

(1) A thickness of 10 cm or more; or

(2) An overlying Ap horizon; or

(3) Cementation in 50 percent or more of each pedon; or

(4) A coarse-loamy, loamy-skeletal, or finer particlesize class and a frigid temperature regime in the soil; or

(5) A cryic temperature regime in the soil; and

b. An upper boundary within the following depths from the mineral soil surface: either

(1) Less than 50 cm; or

(2) Less than 200 cm if the soil has a sandy particle-size class in at least some part between the mineral soil surface and the spodic horizon; and

c. A lower boundary as follows:

(1) Either at a depth of 25 cm or more below the mineral soil surface or at the top of a duripan or fragipan or at a densic, lithic, paralithic, or petroferric contact, whichever is shallowest; or

(2) At any depth,

(a) If the spodic horizon has a coarse-loamy, loamy-skeletal, or finer particle-size class and the soil has a frigid temperature regime; or

(b) If the soil has a cryic temperature regime; and

d. Either:

(1) A directly overlying albic horizon in 50 percent or more of each pedon; or

(2) No andic soil properties in 60 percent or more of the thickness either:

(a) Within 60 cm either of the mineral soil surface or of the top of an organic layer with andic soil properties, whichever is shallower, if there is no densic, lithic, or paralithic contact, duripan, or petrocalcic horizon within that depth; or

(b) Between either the mineral soil surface or the top of an organic layer with andic soil properties, whichever is shallower, and a densic, lithic, or paralithic contact, a duripan, or a petrocalcic horizon. Spodosols, p. 695 

D. Other soils that have andic soil properties in 60 percent or more of the thickness either:

1. Within 60 cm either of the mineral soil surface or of the top of an organic layer with andic soil properties, whichever is shallower, if there is no densic, lithic, or paralithic contact, duripan, or petrocalcic horizon within that depth; or

2. Between either the mineral soil surface or the top of an organic layer with andic soil properties, whichever is shallower, and a densic, lithic, or paralithic contact, a duripan, or a petrocalcic horizon. Andisols, p. 271 

E. Other soils that have either:

1. An oxic horizon that has its upper boundary within 150 cm of the mineral soil surface and no kandic horizon that has its upper boundary within that depth; or

2. 40 percent or more (by weight) clay in the fine-earth fraction between the mineral soil surface and a depth of 18 cm (after mixing) and a kandic horizon that has the weatherable-mineral properties of an oxic horizon and has its upper boundary within 100 cm of the mineral soil surface. Oxisols, p. 655 

F. Other soils that have:

1. A layer 25 cm or more thick, with an upper boundary within 100 cm of the mineral soil surface, that has either slickensides or wedge-shaped peds that have their long axes tilted 10 to 60 degrees from the horizontal; and

2. A weighted average of 30 percent or more clay in the fine-earth fraction either between the mineral soil surface and a depth of 18 cm or in an Ap horizon, whichever is thicker, and 30 percent or more clay in the fine-earth fraction of all horizons between a depth of 18 cm and either a depth of 50 cm or a densic, lithic, or paralithic contact, a duripan, or a petrocalcic horizon if shallower; and 3. Cracks2 that open and close periodically. Vertisols, p. 783 

G. Other soils that:

1. Have:

a. An aridic soil moisture regime; and

b. An ochric or anthropic epipedon; and

c. One or more of the following with the upper boundary within 100 cm of the soil surface: a cambic horizon with a lower depth of 25 cm or more; a cryic temperature regime and a cambic horizon; a calcic, gypsic, petrocalcic, petrogypsic, or salic horizon; or a duripan; or

d. An argillic or natric horizon; or

2. Have a salic horizon; and

a. Saturation with water in one or more layers within 100 cm of the soil surface for 1 month or more during a normal year; and

b. A moisture control section that is dry in some or all parts at some time during normal years; and

c. No sulfuric horizon that has its upper boundary within 150 cm of the mineral soil surface. Aridisols, p. 329 

H. Other soils that have either:

1. An argillic or kandic horizon, but no fragipan, and a base saturation (by sum of cations) of less than 35 percent at one of the following depths:

a. If the epipedon has a sandy or sandy-skeletal particle-size class throughout, either:

(1) 125 cm below the upper boundary of the argillic horizon (but no deeper than 200 cm below the mineral soil surface) or 180 cm below the mineral soil surface, whichever is deeper; or

(2) At a densic, lithic, paralithic, or petroferric contact if shallower; or

b. The shallowest of the following depths:

(1) 125 cm below the upper boundary of the argillic or kandic horizon; or

(2) 180 cm below the mineral soil surface; or

(3) At a densic, lithic, paralithic, or petroferric contact; or

2. A fragipan and both of the following:

a. Either an argillic or a kandic horizon above, within, or below it or clay films 1 mm or more thick in one or more of its subhorizons; and

b. A base saturation (by sum of cations) of less than 35 percent at the shallowest of the following depths:

(1) 75 cm below the upper boundary of the fragipan; or

(2) 200 cm below the mineral soil surface; or

(3) At a densic, lithic, paralithic, or petroferric contact. Ultisols, p. 721 

I. Other soils that have both of the following:

1. Either:

a. A mollic epipedon; or

b. Both a surface horizon that meets all the requirements for a mollic epipedon except thickness after the soil has been mixed to a depth of 18 cm and a subhorizon more than 7.5 cm thick, within the upper part of an argillic, kandic, or natric horizon, that meets the color, organic-carbon content, base saturation, and structure requirements of a mollic epipedon but is separated from the surface horizon by an albic horizon; and

2. A base saturation of 50 percent or more (by NH4OAc) in all horizons either between the upper boundary of any argillic, kandic, or natric horizon and a depth of 125 cm below that boundary, or between the mineral soil surface and a depth of 180 cm, or between the mineral soil surface and a densic, lithic, or paralithic contact, whichever depth is shallowest. Mollisols, p. 555 

J. Other soils that do not have a plaggen epipedon and that have either:

1. An argillic, kandic, or natric horizon; or

2. A fragipan that has clay films 1 mm or more thick in some part. Alfisols, p. 163 

K. Other soils that have either:

1. One or more of the following:

a. A cambic horizon with its upper boundary within 100 cm of the mineral soil surface and its lower boundary at a depth of 25 cm or more below the mineral soil surface; or

b. A calcic, petrocalcic, gypsic, petrogypsic, or placic horizon or a duripan with an upper boundary within a depth of 100 cm of the mineral soil surface; or

c. A fragipan or an oxic, sombric, or spodic horizon with an upper boundary within 200 cm of the mineral soil surface; or

d. A sulfuric horizon that has its upper boundary within 150 cm of the mineral soil surface; or

e. A cryic temperature regime and a cambic horizon; or

2. No sulfidic materials within 50 cm of the mineral soil surface; and both:

a. In one or more horizons between 20 and 50 cm below the mineral soil surface, either an n value of 0.7 or less or less than 8 percent clay in the fine-earth fraction; and

b. One or both of the following:

(1) A salic horizon or a histic, mollic, plaggen, or umbric epipedon; or

(2) In 50 percent or more of the layers between the mineral soil surface and a depth of 50 cm, an exchangeable sodium percentage of 15 or more (or a sodium adsorption ratio of 13 or more), which decreases with increasing depth below 50 cm, and also ground water within 100 cm of the mineral soil surface at some time during the year when the soil is not frozen in any part. Inceptisols, p. 489 

L. Other soils.

Entisols, p. 389

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